A gas turbine includes a compressor, a combustor and a turbine. The compressor generates high-temperature and high-pressure compressed air by compressing air taken in from an air inlet port. The combustor generates high-temperature and high-pressure combustion gas by supplying fuel to the compressed air and burning them. The turbine includes a casing having an exhaust passage in which a plurality of turbine nozzles and turbine rotor blades are alternately arranged. The combustion gas supplied to the exhaust passage is used to drive the turbine rotor blades, thereby, for example, rotatably driving a rotor connected to a generator. The combustion gas having driven the turbine is converted to static pressure by a diffuser provided downstream of the turbine and released to the atmosphere.
In recent years, combined cycles in which a steam generator and a steam turbine are combined and provided downstream of a gas turbine, for increasing the temperature of the gas turbine and improving the output and the efficiency, have been known. Some combined cycles extract steam from the steam turbine and cool a combustor in the gas turbine.
To improve the combined efficiency (heat efficiency) in such combined cycles, it is preferable to reduce the amount of heat exchange to cool at a transition piece outlet of the combustor connected to the turbine. In other words, the heat quantity for cooling the combustor is collected by steam having performed heat exchange, but if the heat quantity to be cooled can be reduced from the beginning, the combined efficiency is improved. If the flow velocity of the combustion gas is slowed by increasing the cross-sectional area of the transition piece outlet, the heat-transfer coefficient is also decreased, thereby reducing the amount of heat exchange. However, at a first stage turbine nozzle of a turbine that is connected to the transition piece outlet and that receives the combustion gas, the size of the first stage turbine nozzle in the radial direction provided downstream (outlet side) of the nozzle is aerodynamically determined. Accordingly, a problem arises when the cross-sectional area of the transition piece outlet is increased.
For example, Patent Document 1 discloses a gas turbine in which the size of an opening provided upstream of the nozzle in the radial direction is larger than that of the opening provided downstream.
In the above-described gas turbine, the combustion gas of the combustor is supplied to a turbine from the first stage turbine nozzle. A combustion gas passage is formed in a ring shape, because the first stage turbine nozzle is arranged along the circumference about the axial center of the rotor. A plurality of combustors is arranged along the circumference about the axial center of the rotor to supply combustion gas to the turbine. Ideally, the center line of each of the combustors is arranged in parallel with the axial center of the rotor, and the combustion gas is ejected straight to the turbine. However, due to structural constraints in the gas turbine casing, the center line of the combustor is obliquely arranged (at least 30 degrees) relative to the axial center of the rotor, so that the combustion gas is ejected obliquely from the outside in the radial direction to the inside in the radial direction. In the combustor, high-temperature and high-pressure combustion gas is produced by taking in compressed air, supplying fuel to the compressed air from a fuel nozzle, and burning them.
For example, as disclosed in Patent Document 2, the combustor includes a transition piece. The transition piece guides combustion gas ejected from the fuel nozzle to a transition piece outlet connected to the first stage turbine nozzle of the turbine from a transition piece inlet immediately after the fuel nozzle. The transition piece is formed to have a circular cross section of the transition piece inlet, and an arc cross section of the transition piece outlet with the ring-shaped combustion gas passage of the first stage turbine nozzle divided by the number of combustors to supply the combustion gas to the first stage turbine nozzle. In other words, the cross-sectional shape of the transition piece is changed from the transition piece inlet to the transition piece outlet. To stabilize the flow of the combustion gas, the transition piece needs to be contracted from the transition piece inlet to the transition piece outlet.